In medical diagnosis, nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) requires the production of a very strong static primary magnetic field for passage through a patient's body. A gradient magnetic field varying with time is superimposed on the primary field. Additionally, the patient is exposed to RF electromagnetic waves that are varied in time and in particular patterns. Under the influence of the magnetic and RF waves, nuclear spin distributions of atomic nuclei can be observed. This technique permits soft tissue and organs of the body to be examined for abnormalities such as tumors.
In MRI, the magnetic field must typically be a strong field on the order of about one kilogauss or more. Fields in excess of ten kilogauss (one Tesla) are sometimes required. Additionally, the field must be uniform, with non-uniformities of no more than one hundred (100) ppm. In addition, this uniformity must encompass a large portion of the patient's body, preferably with a diametral spherical volume (DSV) on the order of about 0.3 to 0.5 meters.
In the past these strong magnetic fields have been generated using permanent magnets, resistive magnets or superconducting magnets. Permanent magnets are typically the least expensive, require minimal site preparation, and are low cost to maintain because they require no liquid cryogens. Permanent magnets however, have limited field strength, temporal instabilities, are very heavy, and are costly at field strengths above 0.20 Tesla. Resistive magnets are also relatively inexpensive but require an elaborate and costly power and water supply. In addition the strength of resistive magnets is limited, large unwanted fringe fields are often generated, and temporal instabilities exist. Superconducting solenoidal magnets have the advantage of a strong field with high uniformity and good temporal stability. Superconducting solenoidal magnets, however, are expensive to construct and maintain and require elaborate cryogenic support structures. In the construction of medical MRI magnets, two different configurations or embodiments are generally in use. One type of structure is known in the art as an open access structure. Such a structure typically includes opposite parallel magnetic pole faces mounted on opposite parallel support plates. At least one and usually four support columns support the support plates and provide a return path for magnetic flux. Such an open structure is favored by patients because it is open and accessible from four sides. With such a structure, the magnetic flux lines pass generally orthogonally to the longitudinal (i.e head to toe) axis of the patient.
Another type of MRI magnet, rather than being constructed with opposite magnetic pole faces or as an open access structure, is similar to a large conventional solenoid. The solenoid structure is generally cylindrical in shape and is helically wound with electrically conducting wire. An electric current conducted through the wire produces lines of flux that run through a central opening of the cylinder and generally coincident with a longitudinal axis of the patient. Such an enclosed solenoid structure is known to give some patients a claustrophobic reaction. In addition, with such an enclosed structure there is no access for additional medical equipment and personnel for performing techniques such as interventional radiology.
Different types of magnet systems have been proposed for use with each of these structures. In the past, open access structures have typically been constructed with permanent magnets attached to the opposite pole faces. U.S. Pat. No. 4,943,774 to Breneman et al. for instance, discloses such an open access MRI structure that utilizes permanent magnets. The supporting structure is fabricated of a ferromagnetic material such as high quality structural steel.
The enclosed solenoid type structures may be formed with superconducting magnets. Such superconducting magnets must be cooled to a temperature close to absolute zero (-273.degree. C.) in order for the wiring to lose resistance to the flow of electric current. Relatively small diameter wires can thus carry large currents and create high magnetic fields. The superconducting wires are typically wrapped around the outer periphery of the cylindrical structure enclosed in a cryostatic vessel. Such an enclosed solenoid type structure may employ a pair of main superconducting coils and one or more auxiliary coils.
With either an open access magnet or a solenoid type magnet it is necessary to provide a uniform and homogeneous magnetic field in the (DSV) formed in the patient receiving area (i.e all lines of flux need to be substantially parallel to one another). One cause of a reduction in the uniformity of the magnetic field in the (DSV) is the production of localized linear gradients by the gradient coils used to produce the gradient magnetic field. In general the gradient coils produce local linear gradients in the x,y, and z directions within the (DSV) to ultimately provide spatial identification within the (DSV). The gradient coils are parallel, and are typically located a few centimeters from the large flat ferromagnetic pole faces utilized in an open access MRI magnet.
Unfortunately, when these gradient coils are pulsed during the utilization of the MRI system, the pole faces of the MRI device become additionally magnetized so that a substantial remnant gradient in the x,y, or z directions exists. Whenever a closed electrical path is exposed to a time varying magnetic field, an electrical eddy current is introduced into the path. These eddy currents generally have a relatively short time constant and act in a way to oppose the changing magnetic field. It is known that eddy currents flowing in ferromagnetic materials can cause magnetic hysteresis. Thus, such remnant gradients add unwanted gradients to the field and degrade the homogeneity of the magnetic field in the (DSV) and further impair image quality. Furthermore it is well known that minimizing eddy current signature, i.e. reducing the time constant and amplitude associated with the transient response, results in beneficial performance of an MRI magnet.
U.S. Pat. No. 4,980,641 to Breneman et al. is directed to a method and apparatus of reducing such hysteresis in an open access MRI magnet that utilizes permanent magnets. In this patent it is disclosed that by providing a shielding layer of nonferromagnetic conducting material on the pole faces, eddy currents are transferred from the pole face to the nonferromagnetic layer, thus minimizing any induced hysteresis. Conducting nonferromagnetic materials include aluminum (Al) or copper (Cu) plates. This patent further discloses that radial slits can be formed on the pole faces to provide a reduced path length for the eddy currents. Using the shielding layer or a slitted pole face structure, unwanted eddy current effects can be minimized so that the time constant is shortened considerably, on the order of one millisecond or less. Moreover, the amplitude of the eddy current, voltage and current signature are reduced to a very small value thus not introducing unwanted remnant gradients into the MRI system.
In light of the shortcomings of permanent magnet systems and of the closed and restrictive structure of conventional superconducting MRI magnets the present invention is directed to an open access MRI magnet constructed with superconducting magnets to generate a strong magnetic field. Such an open access MRI magnet maintains all of the advantages of the superconducting magnet (i.e. strong magnetic field, good temporal stability) and is formed with a patient receiving area accessible from several sides by additional medical personnel and equipment. In addition the superconducting MRI magnet of the invention includes an apparatus for reducing magnetic hysteresis. Accordingly, it is an object of the present invention to provide an open access superconducting MRI magnet having an apparatus for minimizing hysteresis induced by the pulsed gradient coils. It is a further object of the present invention to provide a superconducting MRI magnet for achieving a highly uniform magnetic field. It is yet another object of the present invention to provide an open access superconducting MRI magnet having an apparatus for minimizing hysteresis that is accessible for additional medical equipment and personnel for performing medical techniques such as interventional radiology.